Infrared spectroscopyAQA A-Level Chemistry: Flashcards

Exam questions on Infrared spectroscopy

1. A student records the infrared spectrum of an organic liquid with the molecular formula C₃H₆O. The spectrum has a strong absorption at 1715 cm⁻¹ and no broad absorption anywhere between 2500 and 3600 cm⁻¹. Data: C=O 1680–1750 cm⁻¹; O–H (alcohols) 3230–3550 cm⁻¹; O–H (carboxylic acids) 2500–3000 cm⁻¹; C=C 1620–1680 cm⁻¹.
   Explain how the spectrum shows that the compound is neither an alcohol nor a carboxylic acid.2 marks
2. A pharmaceutical chemist checks the purity of a solid drug product that was made in a reaction using ethanol as the solvent. The drug molecule contains no O–H bond. The chemist compares the infrared spectrum of each batch with the spectrum of a pure reference sample. One batch has an extra broad absorption at 3350 cm⁻¹. Data: C–H 2850–3100 cm⁻¹; O–H (alcohols) 3230–3550 cm⁻¹; O–H (carboxylic acids) 2500–3000 cm⁻¹; C=O 1680–1750 cm⁻¹.
   Explain how the spectrum shows that this batch is impure and what the impurity is likely to be.2 marks
3. Two isomers, P and Q, both have the molecular formula C₃H₆O₂. The infrared spectrum of P has a very broad absorption from 2500 to 3000 cm⁻¹ and a strong absorption at 1710 cm⁻¹. The spectrum of Q has a strong absorption at 1740 cm⁻¹ and a strong absorption at 1200 cm⁻¹, but no broad absorption above 2500 cm⁻¹. Data: C=O 1680–1750 cm⁻¹; C–O 1000–1300 cm⁻¹; O–H (carboxylic acids) 2500–3000 cm⁻¹; O–H (alcohols) 3230–3550 cm⁻¹.
   Deduce the functional group present in P and in Q, referring to the bonds responsible for the absorptions.3 marks